Platelet-derived growth factors (PDGFs) are potent mitogens that exist as five different dimeric configurations composed of four different isoform subunits: A, B, C and D. The five dimeric forms of the PDGFs are AA, BB, AB, CC and DD, which are formed by disulfide linkage of the corresponding individual PDGF monomers. PDGF ligands exert their biological effects through their interactions with PDGF receptors (PDGFRs). PDGFRs are single-pass, transmembrane, tyrosine kinase receptors composed of heterodimeric or homodimeric associations of an alpha (a) receptor chain (PDGFR-alpha) and/or a beta (β) receptor chain (PDGFR-beta). Thus, active PDGFRs may consist of αα, ββ or αβ receptor chain pairings. PDGFRs share a common domain structure, including five extracellular immunoglobulin (Ig) loops, a transmembrane domain, and a split intracellular tyrosine kinase (TK) domain. The interaction between dimeric PDGF ligands and PDGFRs leads to receptor chain dimerization, receptor autophosphorylation and intracellular signal transduction. It has been demonstrated in vitro that ββ receptors are activated by PDGF-BB and -DD, while αβ receptors are activated by PDGF-BB, -CC, -DD and -AB, and aa receptors are activated by PDGF-AA, -BB, -CC and -AB (see Andrae et al. (2008) Genes Dev 22 (10):1276-1312).
PDGF signaling has been implicated in various human diseases including diseases associated with pathological neovascularization, vascular and fibrotic diseases, tumor growth and eye diseases. Accordingly, inhibitors of PDGF signaling have been suggested for use in a variety of therapeutic settings. For example, inhibitors of PDGFR-beta have been proposed for use in treating various diseases and disorders. (Andrae et al. (2008) Genes Dev 22 (10):1276-1312). PDGFR-beta inhibitors include non-specific small molecule tyrosine kinase inhibitors such as imatinib mesylate, sunitinib malate and CP-673451, as well as anti-PDGFR-beta antibodies (see, e.g., U.S. Pat. Nos. 7,060,271; 5,882,644; 7,740,850; and U.S. Patent Appl. Publ. No. 2011/0177074). Anti-ligand aptamers (e.g., anti-PDGF-B) have also been proposed for therapeutic applications. Nonetheless, a need exists in the art for new, highly specific and potent inhibitors of PDGF signaling.